This invention relates to a discharge lamp lighting circuit which reduces the number of parts and costs by improving the configuration of a DC power supply circuit and a DC-AC conversion circuit making up the parts of a discharge lamp lighting circuit.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known.
For example, in the circuit configuration wherein a DC-DC converter is used as a DC power supply circuit and a full-bridge circuit comprising two pairs of semiconductor switching elements for performing switching control and a driver circuit thereof are used for a DC-AC conversion circuit, the positive-polarity voltage (positive voltage) output by the DC-DC converter is converted into rectangular-wave voltage in the full-bridge type circuit, then this voltage is supplied to a discharge lamp. That is, when positive voltage is supplied to one terminal of the discharge lamp, the potential of the other terminal becomes a ground level; and when the potential of the one terminal drops to the ground level, positive voltage is supplied to the other terminal. This cycle is repeated.
However, the lighting circuit requires the four switching elements and the two half-bridge drivers for driving the switching elements as the components of the full-bridge type circuit, causing an increase in the number of parts and a rise in costs; this is a problem.
For example, to use a discharge lamp as a light source of a car""s front light, if a front light is mounted to each of the left and the right of the front of the vehicle, two left and right discharge lamps and their respective lighting circuits become necessary. To adopt a configuration wherein high and low beams are provided by separate discharge lamps (so-called four-light illumination), two left and two right discharge lamps and their respective lighting circuits are required. In such a case, eight switch elements and four half-bridge drivers in all are required for the two discharge lamps, thus costs are increased and in addition, as a unit is upsized, it becomes difficult to provide a circuit unit placement space.
It is therefore an object of the invention to simplify a DC-AC conversion circuit forming a part of a discharge lamp lighting circuit, thereby reducing costs and miniaturize a unit.
To the end, according to the invention, there is provided a discharge lamp lighting circuit comprising a DC power supply circuit for receiving DC input voltage and outputting any desired DC voltage and a DC-AC conversion circuit being placed at a stage following the DC power supply circuit for converting the output voltage thereof into AC voltage and then supplying the AC voltage to a discharge lamp, in that positive-polarity voltage and negative-polarity voltage output separately from two output terminals of the DC power supply circuit are sent to the DC-AC conversion circuit, that a pair of switch elements provided in the DC-AC conversion circuit to switch the output voltages of the DC power supply circuit is alternately operated by a drive circuit of the switch elements, and that AC voltage generated as the switch elements are alternately operated is supplied to the discharge lamp.
According to the invention, a pair of switch elements is provided in the DC-AC conversion circuit for one discharge lamp and drive control may be performed so as to alternately operating the switch elements, so that the circuit configuration is simplified. Namely, two switch elements and one half bridge driver need only be provided per discharge lamp.